Internet DRAFT - draft-freeman-plasma-requirements
draft-freeman-plasma-requirements
Network Working Group T. Freeman
Internet-Draft J. Schaad
Intended status: Informational Soaring Hawk Consulting
Expires: September 6, 2015 P. Patterson
Carillon Information Security Inc
March 5, 2015
Requirements for Message Access Control
draft-freeman-plasma-requirements-11
Abstract
S/MIME delivers confidentiality, integrity, and data origination
authentication for email. However, there are many situations where
organizations also want robust access control applied to information
in messages. The Enhanced Security Services (ESS) RFC5035 for S/MIME
defines an access control mechanism for email, but the access check
happens after the data is decrypted by the recipient which devalues
the protection afforded by the cryptography and provides very weak
guarantees of policy compliance. Another major issues for S/MIME is
its dependency on a single type of identity credential, an X.509
certificate. Many users on the Internet today do not have X.509
certificates and therefore cannot use S/MIME. Furthermore, the
requirement to discover the X.509 certificate for every recipient of
an encrypted message by the sender has proven to be an unreliable
process for a number of reasons.
This document presents requirements for an alternative model to ESS to
address the identified issues with access control in order to deliver
more robust compliance for S/MIME protected messages. This document
describes an access control model which uses cryptographic keys to
enforce access control policy decisions where the policy check is
performed prior to the decryption of the message contents. This
authorization model can be instantiated using many existing standards
and is in not intended to be a one off just for email, being
applicable to other data types.
This document also presents requirements for the abstraction of the
specifics of the authentication technologies used by S/MIME users. The
abstraction makes it possible for other forms of authentication
credentials to be used with S/MIME thereby enabling much broader
adoption. The authentication abstraction model also removes the
dependency on the need to discover encryption keys by the sender. This
abstraction can be used independently from access control to enable
simple scenarios where authentication of the recipient is sufficient
to grant access to the message.
The name Plasma was assigned to this effort as part of the IETF
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process. It is derived from PoLicy enhAnced Secure eMAil.
Status of this Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
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Copyright Notice
Copyright (c) 2015 IETF Trust and the persons identified as the
document authors. All rights reserved.
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Table of Contents
1 Policy-Based Management Vocabulary . . . . . . . . . . . . . . . 4
2 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . 6
3. Access Control Models . . . . . . . . . . . . . . . . . . . . . 10
3.1 Generic Access Control Model . . . . . . . . . . . . . . . . 13
4 Use Case Scenarios . . . . . . . . . . . . . . . . . . . . . . . 16
4.1 Consumer-to-Consumer Secure Email . . . . . . . . . . . . . 16
4.2 Business-to-Consumer Secure Email . . . . . . . . . . . . . 17
4.3 Business-to-Business Ad-Hoc Email . . . . . . . . . . . . . 18
4.4 Business-to-Business Regulated Email . . . . . . . . . . . . 19
4.5 Delegation of Access to Email . . . . . . . . . . . . . . . 24
4.6 Policy Compliance Verification . . . . . . . . . . . . . . . 25
4.7 Email Pipeline Inspection . . . . . . . . . . . . . . . . . 25
4.8 Distribution List Expansion . . . . . . . . . . . . . . . . 27
4.9 Scalable Decision Making . . . . . . . . . . . . . . . . . . 27
5 Plasma Security Model . . . . . . . . . . . . . . . . . . . . . 28
5.1 Plasma Client/Server Key Exchange Level of Assurance . . . . 34
5.2 Policy Data Binding . . . . . . . . . . . . . . . . . . . . 34
5.3 Content Creation Workflow . . . . . . . . . . . . . . . . . 37
5.4 Content Consumption Workflow . . . . . . . . . . . . . . . . 38
5.5 Plasma Proxy Servers . . . . . . . . . . . . . . . . . . . . 39
5.6 Policy Types . . . . . . . . . . . . . . . . . . . . . . . . 41
6 Message Protection Requirements . . . . . . . . . . . . . . . . 42
6.1 General Requirements . . . . . . . . . . . . . . . . . . . . 42
6.2 Basic Policy Requirements . . . . . . . . . . . . . . . . . 44
6.3 Advanced Policy Requirements . . . . . . . . . . . . . . . . 45
7 IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 45
8 Security Considerations . . . . . . . . . . . . . . . . . . . . 45
Appendix A. References . . . . . . . . . . . . . . . . . . . . . 47
A.1. Normative References . . . . . . . . . . . . . . . . . . . 47
A.2. Informative References . . . . . . . . . . . . . . . . . . 47
Appendix B Authors' Addresses . . . . . . . . . . . . . . . . . . 49
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Keywords
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119.
1 Policy-Based Management Vocabulary
This document uses the established terminology for policy-based
management [RFC3198] where applicable. The following list supplements
the terms defined in [RFC3198] as well as defining some new
combinations of terms used in [RFC3198].
Attribute Based Where the access control policy is specified
Access Control by a set of attributes, their values, and any
(ABAC) relationship between attributes required to
authorize an action on a resource. These
attributes may be provided by the subject as part
of the decision request (Front-end Attribute
Exchange) or discovered by the policy decision
service itself (Back-end Attribute Exchange). The
policy, for example, may require attributes about
the subject, their device or environment, a
resource, or the intended use of the information.
Back-end Attribute When subject attributes are directly sent from
Exchange (BAE) the Policy Information Point (PIP) to the Policy
Decision and Enforcement Point (PDEP) i.e. they
are not relayed via the Decision Requestor (DR).
Capability Based Where access control is via a communicable,
Access Control unforgeable token. A capability token is a
(CBAC) protected object which, by virtue of its
possession by a subject, grants that subject the
capability.
Decision Requester The service responsible for making policy
(DR) decision requests to the PDEP. In this model the
policy decision is enforced by the PDEP through
its control of cryptographic keys. The DR enforces
any obligations the PDEP may require such as
signing or encryption of the data, generating
audit events etc. A DR is distinct from a PEP in
other models such as XACML in that a DR is not by
default trusted with the clear text data. Policy
enforcement is performed by the PDEP. A DR may
establish trust by presentation of attributes
about itself and its environment to show it is
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trustworthy.
Front-end Attribute When subject attributes are relayed by the DR
Exchange (FEE) from the PIP to the PDEP i.e. they are not sent
directly.
Level of Assurance A quality grade assigned following the completion
(LoA) of a security evaluation. For example, it can be
used for an identity where it provides the quality
of the identity of a subject. It can also be used
to represent the quality of a products or services
Common criteria evaluation.
Metadata Metadata is data about data. There are three kinds
of metadata:
(1) Content metadata is metadata about an instance
of data, the actual data content. An example of
content metadata would be "this data contains
Company Foo intellectual Property" or "this is a
patient record".
(2) Policy metadata is metadata about the policies
to apply to an instance of data. An example of
policy metadata would be "apply Company Foo XYZ
policy".
(3) Structural metadata is metadata about the
design and specification of the data. An example
of structural metadata would be "this is a patient
record table".
Orthonym The correct or legal name of a place, person, or
thing. (See Pseudonym.)
Policy The system entity that creates, maintains, and
Administration publishes policies or policy collections. The
Point (PAP) policies define the rules, their conditions, and
actions associated with the policy.
Policy Collection A collection of one or more policies which is
associated with a role. The policy collection may
also define the logical relationship between the
policies. Each collection is identified by a name
known as a role name.
Policy Decision The system entity that both evaluates the policy
and Enforcement criteria published by a PAP, using attributes
Point (PDEP) supplied by a PIP to render decisions on requests
made by DRs and enforces its decision via the use
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of cryptographic keys.
Policy Decision The system entity that evaluates the policy
Point (PDP) criteria published by a PAP, using attributes
supplied by a PIP to render decisions on requests
made by DRs. The PDP has a separate enforcement
point.
Policy Enforcement The system entity that enforces the decisions of
Point (PEP) a PDP.
Policy Identifier The tag that is used to identify a policy. For the
purposes of this document the focus is on two
different types of policy identifiers. Object
Identifiers (OIDs) are what are currently used in
many security policy systems and are the only
method of policy identification supported by ESS
security labels. Additionally URIs are supported
as policy identifiers as they provide a more
user-friendly method to uniquely identify a policy
and allow discovery of the policy.
Policy Information A service which issues assertions, for example
Point (PIP) about a subject, their device, or environment
e.g., an LDAP directory or SAML Security Token
Service.
Policy Label The data structure which holds one or more policy
identifiers and their logical relationship.
Pseudonym A name that a person or group assumes for a
particular purpose, which differs from their
original or true name. (See Orthonym.)
Role Token A token, issued to a subject, containing one or
more Policy Collections. The role token is used as
part of policy discovery and management in Plasma.
It is not used as part of access control decisions
in any way.
2 Introduction
The S/MIME standard [RFC5751] provides a method to send and receive
secure MIME messages. S/MIME uses CMS[RFC5652] as the means to protect
the message. While CMS allows for many types of key exchange
mechanisms to be used, S/MIME [RFC5750] exclusively uses X.509
certificates [RFC5280] for the security credentials for signing and
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encryption operations. S/MIME also uses an early binding mechanism
for encryption keys where the sender needs to discover the public key
for every recipient of an encrypted message before it can be sent.
This requires the sender to maintain a cache of all potential
recipient certificates (e.g., in a personal address book) and/or have
the ability to find an acceptable certificate for every recipient from
a repository at message creation. This key management model has
limited the use of S/MIME for encryption for a variety of reasons and
is a major factor in the lack of adoption of S/MIME. The S/MIME key
management model has many dependencies resulting in senders often
unable to encrypted email to recipients. For example, to encrypt a
message, the sender needs to discover the X.509 certificate for every
recipient. This may not be possible for a variety of reasons:
o The recipient may not have an X.509 encryption certificate
o The sender may not have previously received a signed email with the
recipient's certificate
o The recipient may not have an available repository from which to
publish their certificate for senders to discover
o The sender may be unaware of the location of the recipient's
repository
o The recipient's repository may not be accessible to the sender,
e.g., it's behind a firewall
o The sender may not have a valid certificate path to a trust anchor
for the recipient's certificate
If one or more recipient certificates are missing, then the sender is
left with a stark choice: send the message unencrypted or remove the
recipients without valid certificates from the message.
The use of secure mailing lists has the ability to provide some relief
to the above problems, especially for cross-domain scenarios. The
original sender only needs to know the appropriate encryption
information for the mailing list in the other domain; the mailing list
in turn, enables its recipients to decrypt the message as part of the
mailing list expansion. It can thus be thought of as a form of late-
binding of recipient information for the originating sender. As a
solution it therefore helps where all recipients have trusted
certificates but the certificates of recipients in other domain are
not discoverable by the sender in their domain. The certificate of the
mailing list is however discoverable by the sender, and it enables all
email sent to the mailing list to be encrypted to named recipients on
the mailing list in all domains.
In many regulated environments end-to-end confidentiality between
sender and recipients by itself is not enough. The regulatory policy
requires some form of access control check before access to the data
should be granted. In many inter-organization collaboration scenarios
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it's impossible for the sender to satisfy the access checks on behalf
of recipients in other organizations since that would require the
sender's client or an agent in the sender's domain to access to those
recipients' attributes to perform the access check, which may be a
breach of the recipients' privacy. Indeed to release the attributes to
the sender may require that the sender's attributes first be released
to the recipients' attributes provider to authorize the release of the
recipients' attributes. It's also a fundamental tenet of good privacy
practice that consent should be obtained from users before release of
data about themselves.
ESS Security labels are an optional security service for S/MIME. The
ESS security label allows classification of the sensitivity of the
message contents using a hierarchical taxonomy in terms of the impact
of unauthorized disclosure of the information [RFC3114]. The security
label can also indicate access control policy. ESS security labels
are authenticated attributes of a CMS signer-info structure in a
SignedData object. The label, when applied to signed clear text data,
provides the access-control requirements for the plain text. If
applied to cipher text such as the outer layer of a triple-wrapped
S/MIME message the label is used for coarse-grained optimization such
as routing.
ESS Security Labels have been found to have a number of limitations.
1. When the label is on the innermost content, access to the plain
text is provided to the recipient (in some form) independent of
the label evaluation as it will be processed for the purpose of
hash computation as part of signature validation. Depending on
how a triple-wrapped message is processed by the recipient's CMS
code, the inner content may be processed for signature validation
even before the outer signature is validated. This would happen
for a stream-based CMS processor which starts processing inner-
layers immediately rather than finishing processing of each layer
and caching the intermediate results.
2. While labels cannot be altered, they can be removed in transit.
If a signed layer is seen then it can be removed by any agent that
processes the message (such as a Message Transfer Agent). If the
label is protected by an encryption layer then it can only be
removed by any agent that has a decryption key (Encryption Mail
List agents or Spam Filtering software would be two such
examples).
3. Policies are identified by Object Identifiers. This makes for a
small tight encoding, but it does not provide any mechanism for an
email client to discover how to enforce an access control policy
if the message contains a policy the client is unaware of. This
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provides an impossible choice: ignore the access control policy
and grant access to the message or block access to the message.
Object identifiers also do not provide a good display name for
users so that they could manually find and download a new policy.
4. The current ESS standard only allows for a single policy label in
a message; no standardized method of composing multiple policy
labels together has been defined. This is adequate for coarse-
grained policy binding to express a limited set of choices such as
with information sensitivity which typically provides a hierarchy
of 3-5 choices. Many data sets need to be subject to multiple
access control policies. For instance, a message may contain
information that is both propriety and export controlled. Trying
to represent combinations of policies via a single policy label
would lead to an exponential growth in the number of policy
labels.
5. ESS Labels do not provide for any robust auditing of who has been
granted access to the message. All policy evaluation is local to
the recipient's machine; no centralized logging of access to the
message can be performed
6. The biggest issue with ESS labels is enforcement of the policy
occurs on the recipient's machine; the compliance with the policy
is dependent on the state of the configuration of every receiving
agent. The policy is enforced by whatever module is located on
the user's system. For cross corporate systems, this means that
the policy provided by Company A must be installed on Company B
machines, or Company B must install a policy that Company A will
accept as being equivalent to their own policy. Additionally, any
time that a new version of the policy module is rolled out, there
will be a time lag before every recipient's machine will have the
updated module. This makes policy compliance practically
impossible in anything but a small, closed environment.
From a regulatory enforcement perspective, ESS labels are an extremely
weak form of access control because cryptographic access to the data
is given before the access check. The correct enforcement of the
access check is dependent on the configuration of every recipient's
email client. Since the cryptographic access is granted before the
access policy check, there is no cryptographic impediment for a
recipient who is able to decrypt the data but is unauthorized under
the policy, to ignore the policy and access the data. A stronger
enforcement model is needed for regulatory control for email where
cryptographic access is only granted after the access check is
successful.
S/MIME today can only use X.509 certificates to protect the
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confidentiality or the data origin authentication and integrity of the
messages. There are many users on the Internet today who have other
forms of authentication credentials. This means the many users without
X.509 certificates cannot use S/MIME. There have been many
developments in authentication technology and best practices since
S/MIME was developed over a decade ago, and example of which is SAML
[SAML-core]. The critical difference between SAML and X.509
certificates is that SAML abstracts the details of the authentication
protocol from the application protocol. The identity provider can use
a broad range of authentication mechanisms via SAML such as passwords,
one-time passwords, biometrics, X.509 certificates, etc., to
authenticate the subject without impacting the relying part or
application protocol. Adopting the abstraction model for S/MIME would
enable almost anybody with any kind of authentication credential
registered with one of the many identity providers on the Internet
today to use S/MIME making it possible that S/MIME use may become as
pervasive as TLS is today.
There are many other non-email use cases which would be subject to the
same access policy requirements. Email allows users to create content
and distribute it to a set of recipients. Similar use cases can be
performed with other data formats or applications such as documents
and instant messages. Policy is tied to the information, not the data
format or application, therefore if an organization has a policy
relating to a type of information, then that same policy would apply
to the same information in any form; email, document, or instant
message. While some aspects of this work will be specific to email,
there will be many which would be reusable in other areas.
3. Access Control Models
Access control is the process whereby systems are able to decide
whether to grant a request to access a resource from a subject. There
are a number of models the system can follow to make the decision.
These are two types of models, those based on a subject attributes and
those based on a subjects capabilities. For models based on subject
attributes, the system obtains a set of attributes about the subject
then applies a policy expression using the attributes as input to the
policy to determine the result. For models based on subject
capabilities, the subject has an unforgeable token or reference to a
token attesting to an access to a resource.
The simplest model based on subject attributes is Discretionary Access
Control (DAC) where subject attributes are the subject's identity and
their group memberships. The access control policy is expressed as an
Access Control List (ACL) which is a list of identities and or groups
together with the allowed (or denied) access for each entry. The ACL
is evaluated sequentially, and the first match is the access control
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decision. Under the newer taxonomy for access control models, DAC is
Identity based Access Control (IBAC) where the access control is based
on the subjects identity or the identity of a group they belong to.
Role Based Access Control (RBAC) is a refinement of DAC where the role
is an abstract identity associated with a specific function which is
granted a set of permissions and a subject can be assigned one or more
roles. When a subject is assigned a new function, they are assigned
the role for that function. The role used to simplify management, in
essence each role is a collection of groups.
Capability Based Access Control (CBAC) which is based on unforgeable
tokens which contain a reference to an object together with the access
permissions to the object. CBAC token were initially implemented as
privileged data structures by the operating system, where possession
of or reference to the token grants the bearer the access rights
defined in the token. Modern cryptographic techniques allow the tokens
to be integrity protected so can be passed more openly between
systems.
Attribute Based Access Control (ABAC), where policies are defined in
terms of arbitrary attributes of the subject, their device or
environment, their intended action on or use of the information. ABAC
requires the definition of the policy in a policy expression language,
e.g., eXtensible Access Control Markup Language [XACML-core]. ABAC
also requires a secure way to exchange arbitrary attributes, e.g., via
the Security Assertion Markup Language [SAML-core] or via an LDAP
directory.
SAML [SAML-core] defines an XML framework for describing and
exchanging assertion tokens containing attributes. The entity issuing
the assertion tokens is a Policy Information Point. The entity
consuming the assertion with the attributes is known as the relying
party (RP). The well-known scenarios for using SAML are:
o Single Sign-On across systems on different platform technology
o Federated Identity between business partners
o Web Services and other standards, e.g., SOAP-based protocols
SAML tokens can be either Bearer Tokens or Holder-of-Key tokens.
Bearer tokens have no cryptographic key and their security is based on
the time between when the token was issued and time it was presented
to the relying party together with the token being issued for use with
the RP. Low-value transactions can use Bearer tokens where possession
of the token alone is considered acceptable for the transaction risk.
Holder-of-Key tokens contain a cryptographic key (either public or
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symmetric) and like X.509 identity certificates the subject proves its
identity to the RP by demonstrating control over the key, e.g.,
signature or HMAC over some data. The RP can therefore have a stronger
proof of identity by the demonstration of possession of cryptographic
keys. SAML can also be used to express attributes about a subject to
an RP where the subject has authenticated to the RP by some means.
To prevent every relying party from having to become familiar with the
specifics of the various types of subject identity proofing,
authentication technologies etc., used by identity providers (IdP),
abstraction frameworks have been developed which are taxonomies used
by the IdPs to classify the level the level of assurance of the
subject identity. The taxonomies allow the resultant LoA of the
identity to be represented by a number (1 to n) where 1 is the lowest
level of assurance and n is the highest defined by the framework. The
framework provides a simple abstraction of the details of:
o Identity proofing and registration of subjects
o Tokens used by subjects for providing electronic identity
o The token management mechanisms
o Protocols used by subjects to employ tokens to authenticate to an
identity provider
o Protocols used by subjects to authenticate and pass attributes to a
relying party
The relying party simply has to determine the LoA required for access
as defined by the framework. The framework ensure consistent
evaluation of the LoA of the identity for the relying party. It also
means the IdP can change some of the specifics e.g. deploy a new
authentication technology, without impacting the relying part of their
policies. These framework have been drafted by industry organizations
and governments. While all of these frameworks may not agree on every
aspect, at a macro level they do exhibit many similarities. A common
theme in many is the adoption of a small number of levels of identity
assurance. A simplified description of the levels is:
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--------------------------------------------------------------
| Level of Assurance | Confidence in the Asserted Identity |
|------------------------------------------------------------|
| Level 1 | Negligible Confidence |
|------------------------------------------------------------|
| Level 2 | Some Confidence |
|------------------------------------------------------------|
| Level 3 | Significant Confidence |
|------------------------------------------------------------|
| Level 4 | High Confidence |
--------------------------------------------------------------
3.1 Generic Access Control Model
The terminology defined in [RFC3198] uses a generic information
model for the actors and the way they relate to each other. This
work extends the generic model in the RFC to accommodate ABAC.
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------------------
| |
| Policy |
| Administration |
| Point |
| |
------------------
----------------- |
| | |
| Policy | | Read
| Information | | Policy
| Point | |
| | |
----------------- v
| | v
| | -----------------
| | Back-end Attribute | |
| | Exchange | Policy |
| ------------------------->>| Decision |
| Issue | Point |
| Attributes | |
| -----------------
| ^
| Front-end Attribute ^ Decision
| Exchange | Request +
v | Attributes
v |
----------------- -----------------
| | Request + | |
| Subject | Attributes | Policy |
| Decision | -------------->>| Enforcement |
| Requestor | | Point |
| | | |
----------------- -----------------
Figure 1 Generic Access Control Model
o Administrators manage and publish policies using the PAP. The
published policies are then available to the PDP
o A decision requestor sends a request together with its attributes
to the PEP
o The PEP sends a decision request to the PDP together with the
subject attributes
o The PDP obtains the necessary policy from the PAP
o The PDP can request additional attributes from the PIP
o The PDP returns the decision responce to the PEP
o The PEP enforces the decision
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This generic model assumes the PEP has control over the data i.e. when
it gets the grant decision, it releases the data to the subject. This
works well in client-server situations like access to a web site or
database where there is a clear trust boundary between the subject and
the PEP with the data. However it does not work well with applications
like email where the data is delivered to the subject prior to the
access check. The model needs to be extended to allow the data to be
encrypted and the access check be performed prior to release of the
decryption key.
A dependency in the model is the reliability of the policy selection
for the request by the PDP. The implementation of the policy selection
process can make either a closed- or open-world assumption. Closed-
world assumes the policy set on the PDP is complete therefore there is
a policy in the store for every request. Open-world assumes the policy
store is incomplete and there is a need to discover new policies as
appropriate. Closed-world implementations work when there is
reasonable control over the sets of data managed by the PEP and
policies known to the PDP. However they result in unreliable results
with mobile data, i.e., if data is received from a partner and an
attempt is made to process it via the recipient's PEP and PDP. There
is no linkage between the distribution of the data and the
distribution of the policies in closed-world models. It is therefore
possible that data will be received for which the matching policy is
not available from the recipients policy store.
Access control models based on subject attributes depend upon the
availability of assertions with attributes about subjects. The model
has the PIP issuing attributes about subjects and the PDP consuming
attributes about subjects. A subject can be a human, a device, or a
service. The subject must have a relationship with the PIP since it
has been through some form of registration process with the PIP. There
is no requirement to have a relationship between the PIP and a PDP.
The PDP must trust the PIP, but not vice versa. This is the same model
as exists with X.509. The subject must have a relationship with the
CA, the RP must trust the certificates issued by the CA, but there is
no requirement for the CA to have any form of relationship or trust
with the RP. Release of subject attributes to a PDP must be under a
policy due to the sensitivity of the data. The subjects themselves can
request and give approval for the release of attributes from the PIP
and relay them to the PDP (Front-end Attribute Exchange). If the
subject has given prior consent, the RP may receive attributes
directly from the PIP(Back-end Attribute Exchange). Subject
attributes are potentially sensitive data and are similarly subject to
access control. SAML has the capability to encrypt sensitive data in
the token. The PIP would also develop policy to regulate the set of
data it would release to a PDP.
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The challenges for S/MIME are therefore:
o How to apply this generic access control model to the email
scenarios so there is convergence with other applications, i.e.,
email access control is not a one-off, vertical solution
o How to ensure the access check is possible prior to the recipient
having access to the clear text so the access check is
sufficiently robust for regulators
o How to abstract the authentication credential technology use from
the S/MIME protocol to enable use of the many forms of
authentication in widespread use today on the Internet.
4 Use Case Scenarios
This section documents some email-based use cases that the new
protocol aims to support. Also included are some related scenarios
where the same underlying theme of consistent policy enforcement
equally applies.
4.1 Consumer-to-Consumer Secure Email
One of the issues that is stopping the use of secure email in
personal mail is the fact that consumers find X.509 certificates
difficult and expensive to obtain and then use - especially across
a set of devices (phone, tablet, workstation). One of the possible
use cases of Plasma is to try and deal with this issue by removing
the dependency on X.509 certificates. The details of the use case
are therefore: Alice wants to send an email message to Bob that
contains sensitive, personal data so she is concerned about
ensuring only Bob can read it. Bob has a strong credential he can
use to identity himself, but it's not an X.509 certificate. Alice
needs to ensure the following:
(a) Only Bob can read the email.
(b) Bob has the ability to verify the email is from Alice.
(c) Bob has the ability to verify the email message has not been
modified since Alice sent it.
The sequence of events could be as follows:
1. Alice composes the email to Bob.
2. Alice's email client allows her to classify the email. Alice
classifies the email as Personal Communication which is a policy
provided by her ISP.
3. Alice's email client knows the protections to apply to a Personal
Communication; it knows to encrypt the message.
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4. Alice's email client sends the recipient list and encryption key
to a server and in return gets a message token.
5. Alice's client attaches the token to the message and sends the
message which is able to flow securely and seamlessly through
existing email infrastructure to Bob. The data is protected
while in transit and at rest.
6. Bob receives the email and sees that it is a secure message.
Bob's email client uses the message token to verify that the
secure message has not been altered.
7. Bob's email client attempts to open and decrypt the email using
the message token. If Bob is on the same ISP as Alice, then the
same username/password as he uses to get his email is likly used
to obtain the needed keys. If Bob is on an ISP that is federated
with Alice's ISP then an infrastructure such as SAML, OpenID,
OAUTH, or ABFAB could be used to validate Bob's identity and
authorize the needed decryption keys to be released.
4.2 Business-to-Consumer Secure Email
There are many examples of business-to-consumer secure email scenarios
where the email could potentially contain sensitive medical or
financial data. This would include doctor-patient, bank-account
holder, medical insurance-insured person, and mortgage broker-customer
communications. This example is illustrative of the many use cases for
business-to-consumer email.
A bank (The Bank of Foo) has determined that it will be using email to
distribute statements to its customer (Bob). The information is
confidential, so any channel of communication the bank selects must
protect Bob's privacy. The bank needs to ensure the following:
(a) Only Bob (or additional owners of the account) can read the email
(b) Bob authenticates with a sufficient level of identity assurance
i.e. LoA=>2. The same identity assurance authentication level
used to do on-line banking would be considered sufficient.
(c) Bob can verify the statement is from his bank.
(d) Bob can verify the statement has not been modified since his bank
sent it.
The sequence of events would be as follows:
1. As part of routine end-of-the-month processing, the bank composes
an email to Bob. They include the statement of balances and
activity either as an attachment or as the body of the message.
2. The statement mailer for the Bank of Foo has been configured to
apply a specific policy to the email.
3. The statement mailer for the Bank of Foo knows the protections to
apply based on the policy; it knows to encrypt and integrity
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protect the message and what level of assurance is required for
the recipient's identity.
4. The protected email is able to flow securely and seamlessly
through existing email infrastructure to Bob. The data is
protected while in transit and at rest.
5. Bob receives the email and sees it is a secure message from the
Bank of Foo. Bob can verify the message has not been altered as
it is signed by his bank. Bob uses the same credential as he
would for on-line banking to prove his identity to the email
system and obtain the keys necessary to decrypt the message.
The same process could be used for any messages sent between the
business or organization and its customers. Thus, messages dealing
with loan applications and changes in bank policies can be sent out in
the same manner, potentially using different policies. In some of
these cases it might be in the bank's interests to record in an audit
trail if and when the keys were handed out on certain emails. For a
statement, the bank would not expect a reply to occur, however, for
other types of messages it should be possible for Bob to reply under
the same level of protection. Bob is able to use the same credential
when sending or replying to a message from the bank, as he uses for
accessing the bank's Web site then the bank has the same assurance of
Bob's identity for all transactions.
4.3 Business-to-Business Ad-Hoc Email
Early in the relationship between two companies, it is frequently
necessary to exchange sensitive information as a preliminary to a more
formal business relationship, e.g., for contract negotiations. This
level of security is similar to guarantees to the security afforded by
mail, i.e., you enclose a letter in an envelope which provides a level
of security to the contents while in transit. There is an expectation
that only the recipient or their delegate would open the envelope.
Once the recipient has the letter, you trust them to treat the
contents appropriately.
As an example, Charlie works for Company Foo. He has just met Dave
from Company Bar to discuss the prospect of a potential new business
opportunity. Following the meeting, Charlie wants to send Dave some
sensitive information relating to the new business opportunity.
Charlie trusts Dave to treat the information appropriately. When
Charlie sends the email to Dave with the sensitive content, he must
ensure the following objectives:
(a) Only Dave or his delegate can read the email.
(b) Dave or his delegate is required to authenticate with a LoA => 2
(c) That Dave can verify the email is from Charlie.
(d) That Dave can verify the email has not been tampered with.
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(e) Charlie may also need to keep a record of the fact that Dave
accessed the message and when it was done.
The sequence of events Charlie would use is as follows:
1. Charlie composes the email to Dave. He include some sensitive
information relating to potential terms and conditions for the
new contract that Foo and Bar would sign to form a partnership
for the business opportunity.
2. Charlie's email client allows him to classify the email. He
classifies the email as an ad-hoc pre-contractual communication.
3. Charlie's client knows the protections to apply to ad-hoc pre-
contractual communication; it knows to encrypt and integrity-
protect the message and the level of assurance required for the
recipient's identity.
4. The protected email is able to flow securely and seamlessly
through the existing email infrastructure to the recipient (Dave
in this case). The data is protected while in transit and at
rest.
5. Dave receives the email and sees it is a secure message from
Charlie. (Charlie's policy requires LoA 2 for which Dave uses a
password). Dave is able to prove his identity to the level of
assurance requested by Charlie so he is able to read the email.
The organization Dave works for has an identity service which he
uses to prove his identity for Charlie's email. Dave opens the
email.
4.4 Business-to-Business Regulated Email
As business relationships mature they often result in a formal
contractual agreement to work together. Contractual agreements
would define a number of work areas and deliverables. These
deliverables may be subject to multiple corporate and/or
regulatory policies for access control, authentication, and
integrity. Some classes of email may have information which is
legally binding or the sender needs to demonstrate authorization
to send some types of messages where authority to send the
message is derived from their role or function. Also many
regulated environments need to be able to verify the information
for an extended period (years). The set of policies applicable
to an email is potentially subject to change as the different
user's contribute information to the email thread.
4.4.1 Regulated Email Requiring a Confidentiality Policy
Company Foo has been awarded a contract to build some equipment
(Program X). The equipment is covered by export control which
requires information only be released to authorized recipients
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under the terms of the export control license. Company Bar is a
foreign subcontractor to Company Foo working on Program X.
Company Foo sets up some business rules for access to Program X
data to ensure compliance with the export control license
requirements. Company Foo also sets up separate rules to cover
the confidentiality of its intellectual property contributed to
Program X. Company Bar also sets up its own policies to protect
the confidentiality of its own intellectual property it
contributes to Program X. As part of the agreement between Foo
and Bar, they have agreed to mutually respect each other's
policies.
Confidentiality policies can change over time. It is important to
be able to implement the changes without the need to update the
data itself to reflect the change as finding all instances of the
data is an intrinsically impossible problem to solve.
Frank is an employee of Company Foo. He has been assigned as a
design team leader on Program X and as an individual contributor
on Program X integration. Frank wants to send some email as a
team leader to colleagues working on Program X in both Companies
Foo and Bar.
Grace is an employee of Company Bar. She has also been assigned
to the design team of Program X.
When Frank sends the email with Program X regulated content he
must ensure compliance with all applicable policies based on the
message contents. When Frank sends a Program X email he must
ensure recipients are authorized to read the contents to ensure
Company Foo remains in compliance with all necessary policies
license.
If Frank also includes Company Foo intellectual property in an
email, he must also ensure recipients are authorized to read the
intellectual property contents.
When Grace receives a Program X email, she must provide
attributes about herself to prove compliance with the export
control policy. If the email also contains Company Foo
intellectual property, she must also provide attributes to show
she is authorized to read the information under the agreement
between Company Foo and Company Bar. Because she does not know
all the details of the policies, Grace would not know the set of
attributes she needs to disclose to the PDEP to access the
message. Grace starts with a basic set of attributes to identify
herself. The PDEP may be able to discover more attributes about
Grace once it knows her identity, and if it unable to find all it
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needs, it can request any missing attributes from Grace.
If Grace sends an email with Company Bar intellectual property,
she must ensure recipients are authorized to read the contents
under the agreement between Company Bar and Company Foo.
When Frank sends a Program X email he must ensure the following
objectives:
(a) All recipients meet the necessary policy requirements based on
the message contents; if the message contains Program X export
control data, they meet that policy; if it contains Company Foo's
intellectual property data, they meet that policy; if it contains
both types of data, then recipients must meet both policies.
(b) Recipients authenticate with an identity assurance as required by
the policies, e.g., LoA 3 or above.
(c) Recipients present all other attributes about themselves
necessary to verify compliance with the applicable policies (e.g.
their program assignment, nationality, professional or industry
certifications, etc.).
(d) Recipients can verify the email is from Frank to the level of
identity assurance as defined by the message policy (i.e., level
3 or above).
(e) Recipients can verify the email has not been tampered with to the
level of identity assurance as defined by the message policy.
(f) Recipients are made aware that the message is a Program X email
(and the contents can only be shared with other Program X
workers) and/or the message contains Company Foo's intellectual
property.
The sequence of events Frank would use is as follows:
(1) Frank composes the email and includes a Program X distribution
list as a recipient. He include some information related to
Program X which is export controlled. Frank also includes some
information which is Company Foo's Intellectual Property.
(2) Frank's email client allows him to select the Program X role. The
client then allows Frank to select from a set of policies
appropriate for Program X.
(3) Frank selects the Program X export control content and Company
Foo IP policies from the list of available policies.
(4) The email client knows to encrypt the message, the key size, and
algorithm to use. It also knows, based on the policies selected,
that the message needs to be signed with a LoA 3 or above private
key.
(5) Frank clicks the "send email" button. The client signs the email
using his smart card private key and includes the certificate
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with the appropriate public key for verification of the signature
by recipients. The client then encrypts the message and obtains a
token for the message from a server that will enable the
recipients' servers to enforce the access control requirements
for Frank, and sends the email to his email server.
The email is able to flow securely and seamlessly through existing
email infrastructure to recipients of the distribution list. Grace is
on the distribution list so she receives the email from Frank.
(6) Grace receives the email. Grace's client provides the attributes
necessary to comply with the policy which includes her level 3
encryption certificate to the PDEP.
(7) Once Grace has shown she passes the policy requirements, the PDEP
releases the message Content Encryption Key (CEK) to Grace using
her level 3 encryption certificate.
(8) Grace uses her smart card to open the message. She sees the
message is signed by Frank and marked with both the Program X and
Company Foo IP policies.
(9) The CEK Grace received has a Time To Live (TTL) value which
defines when Grace must discard the CEK and reapply for a new
CEK.
Grace is able to open the message and can reopen the message without
reauthorization within the time window of the TTL. Once the TTL
expires, she must discard the CEK. If she needs subsequent access, she
must reauthorize her access to the message in the same way as when it
first arrived. If some of her attributes change, e.g., Grace is
removed from Program X by Company Bar, then she may not get access to
the message in the future.
If Grace replies to the email from Frank, because Company Bar has
decided agreed to enforce the Company Foo policies for protecting
Company Foo data, the new message inherits the policy from the
original message. If Grace includes some information which is Company
Bar's IP she also adds her company's IP protection policy requirements
to the message.
Frank receives the reply from Grace. Frank is able to prove his
identity to the level requested by Grace and provides the requested
attributes about himself to satisfy both the Program X export control,
the Company Foo IP protection policies, as well as the Company Bar IP
protection policies. Frank opens the email.
The policy also applies to messages forwarded by Frank and Grace
because they contain information from Company Foo and Company Bar and
both companies wants consistent policy enforcement on their
information and have chosen to use each other policies as a means to
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protect each other's data.
After some time, Company Bar fails an audit to show they are complying
with all the requirements for Program X. As a result, Company Foo
updates its policies for Program X to remove Company Bar as an entity
approved to access Program X data. All company Bar employees will then
no longer be able to receive new CEKs for Program X emails as they can
no longer satisfy the Program X policy requirements. Existing CEKs
should be discarded after the expiration of the TTL.
4.4.2 Regulated Email Requiring an Integrity Policy
Company Foo has been awarded a contract to build some equipment
(Program X). This equipment is regulated by the National Aviation
Authority (NAA) that has oversight of Company Foo. The NAA requires
strict procedures at a number of significant events for Program X such
as in the design and maintenance of Program X (e.g., when a design is
complete and released to manufacturing). The sign-off process requires
personnel be suitability qualified and that the documentation needs to
be maintained for the service life of the project (25 years for
Program X).
Company Foo has instigated an email-based sign-off procedure to
simplify sign-off and reduce costs. At the appropriate time, a sign-
off request policy email is sent to the designated program members. If
recipients want to approve, they reply to the request using the sign-
off-approved policy as required by the sign-off request policy.
Frank is the lead on the Program X design team. They have a design
which they believe can be released to the integration team. Frank
initiates the sign-off process for the design.
Grace is one of the sign-off design team members for Program X. She
receives the sign-off request email. Grace responds and applies the
sign-off approved policy to the email. The policy requires Grace to
authenticate with the required level of identity assurance, present
attributes about herself, her device, her work effort assignments and
professional qualifications, and attributes about the data being
approved to demonstrate compliance with the policy. The sign-off
approved policy has some options for how to communicate attributes
about the data being approved. It can require Grace to send a hash of
the data over a secure transport, or she can send a signed hash of the
data if she has a suitable signing certificate, she can send the data
itself. The policy will define what options are acceptable.
The message is signed to indicate Grace met the policy.
When Frank initiates a Program X sign-off email, the system must
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ensure the following objectives:
(a) Frank was authenticated to the level of identity assurance
required under the policy to initiate the sign-off process.
(b) Frank possessed the necessary attributes as required by policy to
initiate the sign-off process.
(c) The contents of the email have not been modified to the level of
assurance required by the policy.
(d) Frank was made aware that is this a sign-off request email and
confirms he intended to initiate the sign-off process.
(e) The state of Frank's system was known to the level of assurance
required under the policy to be free from agents which might
interfere with the sign-off process.
(f) Recipients of the approval message can confirm over the lifetime
of the program, that the signature was applied because the sender
of the approval message complied with the approval policy at the
time of the signature. The confirmation of the policy approval
does not require knowledge the specifics of the policy.
The sequence of events Grace would use is as follows:
(1) Grace receives the sign-off request email.
(2) Grace replies to the email and completes the form data in the
email to show she is approving the sign-off.
(3) Grace clicks the send button to send the email.
(4) Grace receives a sign-off confirmation dialogue before the email
is sent where she is able to confirm her intent is to approve the
sign-off of the component.
(5) Grace's system submits the decision request to send the sign-off
email. Her system is asked to provide attributes about Grace, the
state of her system and the data being authenticated as part of
the decision request.
Grace would not know the complete set of attributes required to submit
her sign-off as she does not know what the policy requires and would
start with a basic set to identify herself. The PDEP may be able to
discover additional attributes about Grace, and if it is still missing
some, can request those missing attributes from Grace. If Grace's
request meets the policy, her system receives a signed statement from
the PDEP that the message meets the policy which is attached to the
email and the message is sent.
4.5 Delegation of Access to Email
There are a number of times when others are given access to a
recipient's mailbox or email is forwarded to other recipients based on
the original recipient's rules. This may be a long-standing
relationship such as when an assistant is given access to an
executive's mailbox. Alternatively, it may be a temporary relationship
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due to short-term needs (e.g., to cover for a vacation). There are
also organizational role mailboxes where the recipient is a role and
one or more users are assigned to the role.
Grace is going on vacation. While Grace is away, Brian will act as a
delegate for Grace. Grace configures a mailbox rule to forward Program
X email to Brian for the duration of her vacation. Brian is able to
satisfy the policy requirements for the Program X email as outlined
above and is therefore able to open the protected email sent to Grace.
Frank does not need to take any actions to allow Brian to access the
email.
4.6 Policy Compliance Verification
Verification is an essential part of compliance. Verification of
compliance with a policy may be conducted by internal staff or
external auditors. The verification need to confirm that the policy
rules are being enforced, e.g., when data is accessed. Auditing
relies on the generation of artifacts to capture information about
events. Typically, this is done via some form of logging. A challenge
here is that for a distributed system and data, the set of logs which
completely describes the transaction are scattered across many systems
so consistency of the audit settings and correlating all the audit
data is problematic. Another consideration is accurately capturing
only the set of desired data, i.e., accurately targeting the set of
events that needs to be logged
Jerry is the compliance officer for Company Foo. He has a procedure
for ensuring compliance for Program X. The procedure defines what to
log and when to audit access to Program X data. Jerry has tools to
collect the audit data and run an analysis to verify the policies are
being followed.
The sequence of events Jerry would use is as follows:
(1) Jerry configures an audit obligation for access to Program X
data. The obligation defines the set of attributes to capture
when Program X data is accessed. The obligation is part of the
Program X policy. Part of the Program X policy is the set of
PDEPs which can process policy decisions on Program X data.
(2) Jerry configures his audit log collection to download Program X
audit log entries from the designated PDEPs.
(3) Jerry also has an audit confirmation tool which "pings" the PDEPs
for access to Program X data. Jerry's audit log analysis tool
looks for these pings to confirm that auditing is taking place as
expected.
4.7 Email Pipeline Inspection
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Organizations have a huge incentive to inspect emails entering or
leaving the organization. Such inspection is desired for many
different reasons. Inspection of mail leaving an organization is
targeted towards making sure that it does not leak confidential
information. It also behooves organizations to check that they are not
a source of malicious content or spam. Inbound mail is checked
primarily for malicious content and phishing attempts as well as spam.
For domains with a high volume of messages there is a strong need to
process email with minimal overhead. Such domains may mandate that
they be pre-authorized to process an email due to the overhead a per-
message request to an external service would add to message
processing.
Company Foo has a policy to scan all inbound and outbound email to
ensure it is free from malware. Company Foo also wants to ensure email
is not spam. Company Foo can own their scanning servers or such checks
may be outsourced to a third party service. Company Foo wants to
ensure that its policy of scanning message contents also applies to
encrypted email.
The ability to decrypt and check the message content for malicious
content is highly desirable. There are a number of methods that can
accomplish this:
1. When a Company Foo client requests to send a Plasma email, the
PDEP is able to check to see if the policy allows email content
inspection by the MTA for this policy, and if it does, that
Company Foo has an outbound email scanning capability, and that
the scanning servers meet the policy requirements. It is able to
pre-authorize the Company Foo email scanning servers to access
the email.
2. The scanning MTA authenticates to the PDEP as an entity doing
virus and malware scanning on a protected message. If the PDEP
has specific policy that allows for access to such a scanning MTA
service, the appropriate decryption keys will be released and the
server will scan the mail and take appropriate action.
3. The policy server is configured with information about various
gateways (both internal and external) and has certificates for
the known gateways. The policy server can then return a normal
X.509 recipient info structure (cryptographic lockbox) to the
sender of the message for direct inclusion in the recipient info
list of the message. This allows normal S/MIME processing by the
scanning MTA without the necessity to query the PDEP server for
keys for specific messages.
4. If the scanning MTA server cannot gain access to the decrypted
content using one of the two proceeding methods, it either passes
the encrypted mail on to the recipient(s) without scanning it or
it rejects the mail. This decision is based on local policy of
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the scanning MTA. If the message is passed to the recipient(s),
then the necessary scanning either will not be done, done by a
downstream MTA, or done on the recipient's system after the
message has been decrypted.
4.8 Distribution List Expansion
A distribution list (DL) is a function of an MTA that allows a user to
send an email to a group of recipients without having to address all
the recipients individually. The membership of the DL may be
confidential so the sender may not know all the recipients. The DL may
be maintained by an external organization. Since a DL is identified by
an email address, the user may be unaware they are sending to a DL.
Plasma policies may have the list of recipients as a parameter of the
policy in the message, thus the fact that the message is being
processed by the distribution list means the MTA processing the
message needs to expand the list of recipients to allow the new
recipients to access the message. Organizations may also require
inbound scanning of email and have thus published keys to enable pre-
authentication of the MTA by the sender to expedite processing. For
both scenarios the DL MTA has to notify the Plasma server that it is
adding recipients to the message and supply the list of new
recipients. The Plasma server can then take appropriate action on the
message token and return an updated token if required.
4.9 Scalable Decision Making
Collaboration involves working with external organizations, e.g.,
partners and suppliers. These collaborations may be short- or long-
lived, with a small or very large number of participants.
Organizations therefore need flexibility in deployment and scaling.
Organizations do not want to be forced into having to provide capacity
themselves for all decision-making over their data. Senders would be
happy to delegate decisions where appropriate to partners or external
services provided those decisions use the rules they define for their
data. Likewise, recipients might be happy to leverage their local
decision capacity providing they don't have to duplicate the rules of
the partners, and can simply and easily use policies published by
their partners. An organization may also want to use cloud-based PDEPs
where appropriate as a cost effective way to add capacity and to be
able to respond to transient capacity fluctuations.
The Program Managers for Program X at Companies Foo and Bar agree to a
series of roles which are used to manage personnel and their assigned
policy groups. The policy administrators for Company Foo and Bar
respectively publish the roles and a policy collection for each role.
There are rules associated with the policy collection, for example
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every role uses the Program X policies published by Company Foo.
Employees from Company Foo also get the Company Foo Intellectual
Property policies for those roles, whereas employees from Company Bar
get the Company Bar intellectual property policies for Program X.
Company Foo has also decided to allow enforcement of Program X
policies by PDEPs in both Company Foo and Company Bar. Company Foo has
also decided to use an export-controlled approved cloud-based decision
engine for Program X to allow lower-cost capacity and scaling. Company
Foo is able to add new instances of the cloud-based decision services
as the program scales up and more uses start working on the program.
Each decision engine dynamically discovers the policies it needs from
the set published by Company Foo and Company Bar. Both Company Foo and
Company Bar can add new policies to the policy collections at any time
and they are dynamically discovered by all the policy decision
engines.
5 Plasma Security Model
A common theme from these scenarios is the need to closely tie the
information asset to the set of technical controls via the data
owner's policies in such a way so it is possible to consistently apply
the technical controls across a broad set of applications (not just
email), for a broad set of users (not just those within an
organization), and in a broad set of environments. Assumptions based
on closed-world, enterprise security models are increasingly breaking
down. Perimeter security continues to diminish in relevance and focus
needs to be shifted to self-protecting data as opposed to protecting
the machines that store such data. The binding between the data and
the applicable policies needs to happen as close to the data creation
time as possible so ad-hoc trust decisions are not required.
The delivery of the documented use cases will require the integration
of many existing and some new protocols. In order to ensure the right
overall direction for Plasma as each part of the work proceeds, a
high-level data model is documented here to act as a guide. While this
is technically informative to the developments of each individual
component, it is normative to the work overall.
This Data Centric Security model is based on a well-established set of
actors for policy enforcement used elsewhere [RFC3198] [XACML-core].
Figure 2 shows the relationship between the actors.
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------------------
| |
| Policy |
| Administration |
| Point |
| |
------------------
|
----------------- | -----------------
| | | | |
| Policy | | Read | Policy |
| Information | | Policy | Information |
| Point | | | Point |
| | | | |
----------------- v -----------------
| | v | |
| |Issue ----------------- Issue | |
| |Attributes | | Attributes| |
| |(BAE) | Policy | (BAE) | |
| -------------->>| Decision |<<--------------- |
| | and | |
| | Enforcement | |
| -------------->>| Point |<<----------- |
| |Protect | | Consume | |
| |Content ----------------- Content | |
| |Request+ Request+ | |
| |Attributes Attributes| |
| |(FAE) (FAE) | |
v | v v
v | v v
----------------- -----------------
| | | |
| Content | Distribute | Content |
| Creation | Content | Consumption |
| Decision | ---------------------------->>| Decision |
| Requestor | | Requestor |
| | | |
----------------- -----------------
Figure 2 General Scheme for Publishing and Consuming Protected Content
The Plasma model is applicable to any type data (email, documents,
databases, IM, VoIP, etc.). This facilitates consistent policy
enforcement for data across multiple applications. Another objective
is to not require the data holder to have access to the plain text
data in order to be able to make decision requests to the PDEP. The
policy decision is complex so the content creation DR in Plasma just
uses policy pointers or labels to indicate the set of policies
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applicable to the content. The content consuming DR dynamically
discovers the PDEPs that are authoritative for the decisions on
protected content in question. The PDEPs dynamically discover the
specifics of a policy from a PAP using the policy references. The
specifics of policy authoring and policy decision logic modules are
matters beyond the scope of this document. It is important to note
that the actors in this model are logical entities and as such can be
combined physically in different configurations.
o The Plasma model uses references to bind the data and the policy.
When information is created, it is encrypted and a list of
policies that must be enforced by the PDEP is bound to the
protected data.
O The Plasma model includes policy discovery capability for
subjects. This enables subjects to interact with one or more
PDEPs to discover the set of policies each PDEP would permit the
subject to use to protect new content. The PDEP issues a plasma
role token to subject which contains one or more policy
collections. Each policy collection is identified by a role name.
Subjects can pick any combination of policies from a policy
collection, but cannot mix policies from different policy
collections.
o The Plasma model is an Attribute-Based Access Control (ABAC)
model where the ABAC policy is specified in terms of a set of
attributes, their values, and their relationships. The policy may
specify attributes about the subject, their device, or their
environment, or attributes about a resource.
o The ABAC policy does not require the subject provide their
orthonym. Subjects could be anonymous or pseudonymous. What is
required is the presentation of a set of attributes that
satisfies the policy.
o The subject can be required to bind the supplied attributes to
the channel with the PDEP to a level of assurance as required by
the PDEP. If the PDEP only requires low assurance, bearer tokens
over TLS would be suitable. If the PDEP requires higher
assurance, then the holder of key tokens over TLS would be
required where the token key is bound to the TLS channel.
o This model also supports Capability-Based Access Control (CBAC)
where security tokens represent a capability to meet a policy.
Once a subject has proven compliance with a policy, they can be
issued a capability token. The client can subsequently present
this capability token in lieu of a token or tokens with the set
of subject attributes. The net result is that the model can
transition to a Capability-Based Access Control because the
capability token is an un-forgeable token of compliance with a
policy. The token can be used with any resource tagged with the
same policy.
o Plasma has a baseline of a secure transport between the DR and
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the PDEP. One of the decisions the PDEP has to make is the level
of assurance on the release of the CEK to the subject. For
example, the PDEP can release a clear text CEK over the secure
transport to the DR. Alternatively, the PDEP could require the
production of a high-assurance X.509 encryption certificate as a
subject attribute to generate an encrypted CEK.
For the purpose of the Plasma work, it is desirable that the DR and
PDEP be clearly defined as separate services which may be on separate
systems. This allows for a generalization of the model and makes it
less dependent on any specific deployment model, policy
representation, or implementation method. It also allows for a greater
degree of control of the PDEP by an organization such that it is
possible to keep all of the PDEP resources directly under its control
and independent of the data storage location.
The base set of information for a Plasma client is as follows:
o The address of one or more Identity Providers(s) able to issue
identity attributes to the subject
o A means to authenticate to the Identity Providers(s)and issue
attributes to the subject
o The address of zero or more Attribute Providers(s) able to issue
additional attributes to the subject
o The address of one or more Plasma PDEPs able to issue role tokens
to the subject to initiate Plasma policy discovery.
From this base set of data, the subject is able to authenticate to
each Plasma PDEP in turn using the identity token from the Identity
Provider and discover the set of assigned roles. Each role has a set
of policies which can be applied to data. A subject may be assigned to
multiple roles and therefore has the ability to select the most
appropriate role for the content being created. Once a role is
selected, the subject is able to choose one or more policies from the
policy collection for that role. Role assignment is dynamic so the
role discovery needs to be done on a regular (but not frequent) basis.
Policy selection during content creation can be either manual or
automatic. A DR may have sufficient context to be able to select the
role and policies for the subject or have some rules that facilitate
policy selection.
The model allows the content creation DR to discover the role
assignments from multiple PDEP, which would allow the subject to
access policies based on roles from within their organization and from
any partner organization due to cross-organizational collaboration.
The PDEPs that are authoritative for the role assignment for a subject
may be different from the PDEP that are authoritative for enforcement
of a policy collection in question. The DR uses the role token to
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authenticate the content creation request. The PDEP will check that
the requested list of policies for the information is a subset of the
policies in the role token. If the set of policies is a subset of the
policies in the role token, then it will issue the policy metadata
token to be attached to the protected data.
The policy metadata token is a signed data structure created by the
PDEP which is bound to the protected data, i.e., it has a detached
signature over the encrypted data. It contains public policy metadata
attributes which are used by the DR. An example of a public policy
metadata attribute is a list of one or more URLs which represent the
PDEPs that can make policy decisions using the policy metadata token.
The DR can submit the decision request to any PDEP in the list. The
policy metadata token also has a confidential payload containing
private policy metadata attributes used by the PDEP to make policy
decisions. An example of a confidential policy metadata attribute is
the list of CEKs for the protected data which would be released to the
DR if it passes the policy checks.
Policy rule processing and distribution is complex, so the Plasma
model does not require policy rules to be distributed to the DR. The
DR submits the policy metadata token as part of the decision request.
The confidential portion of the policy metadata token contains a logic
tree of policy references. The PDEP uses the policy references to
discover the policy rules to apply to the request. The logic tree
defines the relationship between the policies. The tree has a series
of nodes where each node represents a set of policies and the
relationship for the policies at the node, e.g., are they combined via
and AND clause or an OR clause. The pinnacle of the tree represents
the decision from all the policies in the tree. The use of policy
references minimizes any policy maintenance issues relating to the
protected data due to policy updates. The policy rues can be updated
and the new rules discovered on subsequent decision requests.
The DR and PDEP are required to carry out obligations of the policy
such as specific encryption requirements, e.g., key size or algorithm,
data integrity requirements, time-to-live of the CEK, or audit record
creation requirements. It is a matter for the policy on how to
determine if the DR or PDEP is trusted to carry out the obligations.
This could be achieved by device type and state attributes.
The PDEP makes its decisions based on the requested action from the
DR, the policy requirements from the PAP(s), and the information from
the PIP(s) about the subject, the subject's device, and the subject's
environment. The information about the subject may be exchanged
directly between the PIP(s) and the PDEP (Back-end Attribute Exchange)
or indirectly via the DR (Front-end Attribute Exchange) or both. The
content creator can also include attributes in the policy metadata.
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There is no guarantee that identity and attribute providers will
consistently use the same name to identity a specific attribute or
attribute data. For example they may use different schemas to identify
an email address or use localized names to describe job functions or
roles. These kinds of values may be standardized within communities of
interest, but not globally across all identity and attribute
providers. Therefore it is necessary to canonicalize the attribute
names and values before processing by the policy. The attribute name
and value mapping is part of the policy data set, i.e., it is in
addition to the policy processing rules.
--------------- ----------------- -----------------
| | | | | |
| | | Policy | | Policy |
| Policy | | Decision and | | Decision and |
| Decision | | Enforcement | | Enforcement |
| Point | | Point | | Point |
| | | | | |
--------------- ----------------- -----------------
| | |
| T | T |
| TTTTTTT|TTTTTTT |
V V V
V V V
--------------- --------------- ---------------
| | | | | |
| Policy | | Decision | | Decision |
| Enforcement | | Requestor | | Requestor |
| Point | | | | |
| | | | | |
--------------- --------------- ---------------
| | |
T | T | |
TTTTTTT|TTTTTT | |
V V V
V V V
--------------- --------------- ---------------
| | | | | |
| End | | End | | End |
| User | | User | | User |
| Application | | Application | | Application |
| | | | | |
--------------- --------------- ---------------
(a) (b) (c)
Figure 3 Options For Trusted Actors with Data.
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When drawing a line where the actors in the model are full trusted
with the clear text data there are three possibilities (see figure 2).
Figure 2a shows the full trust line between the user application and
the Policy Enforcement Point(PEP). This is the model for current
standard access control mechanism, e.g., XACML [XACML-core]. In 2a,
the PEP has full access to the plain text data. It makes decision
requests to the PDP and if the decision is affirmative, allows the PEP
to release the data to the application. To use figure 2a for secure
email would require every MTA and MUA to be fully trusted with plain
text data which is impossible.
Figure 2b shows the full trust line between the PDEP and the DR. In
2b, the DR only has cipher text data. The data is encrypted with a CEK
and the PDEP has access to the CEK. The PDEP releases the CEK to the
end-user application when access is granted so the application can
recover the plain text. This mode is viable for secure email as it
does not require the MTA to be trusted with the plain text data and
either the MTA or MUA can act as a DR.
In figure 2c, no actor is given full trust. When the data is
encrypted, the CEK is encrypted for each recipient just as S/MIME does
today. The encrypted CEKs are given to the PDEP and the PDEP releases
the encrypted CEK when access is granted. This mode is also viable for
secure email as the sender can use either conventional public key
cryptography or Identity-Based Encryption[RFC5408] to protect the CEK
for each recipient.
5.1 Plasma Client/Server Key Exchange Level of Assurance
There are a number of mechanisms by which a client and server can
exchange CEKs. As a baseline, Plasma is establishing a secure
transport between the client and server via TLS. However the client
may be a proxy acting on behalf of the subject, therefore transporting
a clear text CEK over the TLS transport would expose the key to the
proxy. There also may be a proxy at the server which is terminating
the TLS transports and forwarding the requests to another server which
would mean a clear text CEK sent over the transport would be exposed
to the server proxy. Policies may require a higher level of assurance
that the CEK is not exposed to unauthorized principals. This requires
encrypting the CEK for the subject before transport. This would
further require the client or the server to provide a public key to
the other party to be used to protect the CEK before sending it over
the secure transport.
5.2 Policy Data Binding
There are three ways to bind policy to data:
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o By value. This is where a copy of the machine-readable rule set is
directly associated with the data, e.g., where a file system has an
Access Control List for the file or directory, or where a rights
management agent embeds a copy of the policy expressed in a policy
expression language in the rights-protected data. When an access
request is made to the data, the PDEP compares the access request
to the policy on the data itself.
o By reference. This is where a reference to the policy is directly
associated with the data, e.g., a URI or a URN which identifies the
policy to be enforced or points to where the policy is published.
For example with S/MIME, the ESS label identifies the applicable
policy by an OID. When a decision request is made for access to the
data, the PDP finds the policy based on the identifier and then
compares the access request to the referenced policy.
o By inference. This is where the policy has a target description in
terms of resource attributes the policy applies to. When a decision
request is made, a set of attributes describing the resource which
is the subject of the decision request is included in the request
by a PEP. The PDP then compares the resource attributes to the set
of target descriptions of the policies in its policy store to
determine the set of policies to apply to the request. For example
when an XACML policy is authored, a target description in terms of
the attributes of the resource for the policy is also defined. When
an XACML decision request is made, the PDP finds the policy set to
apply to the request by matching the set of attributes in the
request against the target description associated with the policies
in its store. It then processes the decision request using the
identified policy set.
The chief strength of binding policy by value is its simplicity. The
policy, being local to the data, can easily and quickly be read by the
PDP. The chief weakness in binding policy by value is maintaining
policy over time as binding by value results in the policy being
replicated for every instance of data the policy is applied to. Many
policies have a multi-year life span and over the course of time,
there is a very high probability that the policy would need to be
updated. Given the high number of copies, updating a value-bound
policy has proven to be a very costly and imperfect process both from
an enforcement and audit perspective. This process is complicated by
the fact that because only the result is stored and not an identifier,
it is hard to identify the policy that has to be updated.
The chief strength of binding by reference is that once the reference
is bound to the data, the policies continue to be consistently applied
over time over multiple instances of the data and as the policies
change over time. Another strength of binding policy by reference is
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it has a clear result as to the set of policies the PDEP has to apply.
If the PDEP does not have a policy, the reference allows the PDEP to
discover the missing policy. If the PDEP is unable to access a policy
for whatever reason, it knows to fail the decision request with a
different error, i.e., "don't know", which means the DR can reasonably
try other PDEPs. The chief weakness in binding by reference is adding
or removing policies requires updating the policy metadata. Adding or
removing policies has the same difficulties as maintaining policies by
value.
The chief strength of binding by inference is it can often be applied
to data without impacting the storage format providing the data
already has a rich and well-defined set of metadata such as the
structural metadata of an SQL table. It also allows new policies to be
applied to the data without updating the metadata. Unstructured data
such as documents have the ability to store metadata but the challenge
here is what metadata to capture. The nature of the metadata is also
context specific, e.g., the policy target description required to
match structural metadata from an SQL query would be different from
the policy target description for matching content metadata for a
document. The chief weakness in binding by inference is the
reliability of the matching of the metadata to the policy target
description. There are a number of factors which affects the policy
matching process:
* The set of available metadata varies with different data types
which makes the policy target definition more complex, e.g.,
structured data such as SQL databases have structural metadata
whereas unstructured data such as documents have content metadata.
* There is a relationship between the metadata that needs to be
captured and the policies that need to be enforced. It's therefore
hard to generalize the rules for what metadata is necessary
independent of knowing what metadata policies require.
* The resultant set of policies to enforce for a decision request is
dependent on the PDEP having a complete the set of policies
(closed-world assumption). It is impossible, however, to detect
missing policies based on the request. Likewise, it is also
impossible to detect if erroneous policies have been selected
based on the request. If data moves from store to store and
thereby uses different PDEPs, it's impossible to determine the
correctness of the result of the policy matching process by the
new PDEP.
The Plasma model is choosing to use binding by reference for two
reasons:
1 The overarching need to consistently enforce the policies selected
at creation time over the lifetime of the data. The typical use
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case is that the set of policies to be enforced on the data may
change their rules over time but it is the same set of policies
that are enforced over the lifetime of the data.
2 Data in many cases is mobile and travels between users and
organizations. Any dependency on consistency of the decision
making entity would be difficult to enforce or verify.
5.3 Content Creation Workflow
The content creation DR bootstraps itself via the following
sequence of events:
(1) The content creation DR is configured with the set PIPs and PDEPs
it trusts.
(2) The content creation DR submits a request for a role token to all
the trusted PDEPs. The role token defines the set of roles the
PDEP allows for the subject. The subject is authenticated to each
PDEP and the contents of the plasma role token authorized by each
PDEP via attributes from the PIP(s). The PIP attributes can be
obtained by the PDEP either via front-end (relayed to the PDEP
from the PIP via the subject) or back-end (direct exchange
between the PDEP and the PIP) processing.
(3) The content creation DR receives zero or more roles tokens from
each of the PDEPs. Each role token has a one or more policy
collections defining the set of allowed policies for that role
when creating new content.
The DR is now initialized with a list of roles and role tokens. It is
now ready to create content and request protection of that content
from PDEPs. This role token request process would typically be
performed as part of the application initialization process. Role
tokens can be cached for the duration of the tokens TTL to reduce the
number of times the application has to invoke the role token request
process. When the user wants to create new content, they use the
following sequence of events:
(i) The user creates the new content
(ii) The user selects the appropriate role for the content, then
selects one or more policies from the policy collection that are
applicable to the content. When the content creation process is
complete, the DR:
(iii) Encrypts the content with one or more locally-generated CEKs
(iv) Submits a policy metadata token request to the PDEP together
with the CEK(s), the set of required policies to be applied, the
role token from the PDEP, and the hash of the encrypted content.
The CEK(s) in the request can be either raw key(s) or CEK(s)
encrypted by a KEK if the policy does not allow the PDEP to have
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the ability to access the plain text data.
(v) The PDEP verifies the set of requested policies is a subset of
the policy set in the role token. In addition to the role
token, the PDEP may also require any other attributes from the
subject as defined by policy to process the creation request.
If the request satisfies the policy requirements, the PDEP generates
the encrypted policy metadata which contains the list of policies and
the CEKs. The metadata is encrypted by the PDEP for all the PDEPs
allowed to service decision requests for the data (the content
creation PDEP does not have to be in the set of PDEPs allowed to
access control decisions). The PDEP includes a list of URLs for all of
the PDEPs allowed to process decision requests and the hash of the
protected content as signed authenticated attributes in the policy
metadata token, then it signs the encrypted metadata.
(vi) The PDEP returns the policy metadata token to the DR
(vii) The DR attaches the policy metadata token to the protected
content and distributes the content.
5.4 Content Consumption Workflow
When a user wants to open some protected content they would use the
following workflow:
(a) The DR verifies the certificate in the signed policy metadata
then determines via local policy if it wants to process the
protected information based on the identity of the PDEP.
(b) The DR verifies the signature on the policy metadata token and
the binding to the encrypted data by hashing the encrypted
information and comparing it to the authenticated attribute in
the policy metadata
(c) The DR creates read token request. The request contains the
signed metadata from the content together with one or more
authentication tokens issued by a PIP. The request may also
contain attributes about the request such as the purpose of the
use of the data.
(d) The DR sends the read token request to one of the URL's of the
PDEPs in the authenticated attributes of the signed metadata
(e) The PDEP decrypts the policy metadata, de-references the policy
pointers, and determines the set of rules to apply to the
request based on the policy published by the PAP. The PDEP then
determines the set of attributes it needs to evaluate the policy
rules. The PDEP can use PIPs it has direct relationships with to
query attributes about the subject. If the PDEP is missing
attributes it needs to process the policy, it returns a list of
the missing attributes to the DR.
(f) If the DR receives a list of missing attributes from the PDEP,
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it obtains the missing attributes requested by the PDEP from a
PIP and sends them to the PDEP in a new read token request.
(g) Once the PDEP has a complete set of attributes, and the
attribute values match those required under the access policy,
the PDEP releases the CEK to the DR along with a TTL which
defines how long the DR can use the CEK before it must discard
the CEK and reapply for access.
(h) Once the DR has the CEK it decrypts the information. It caches
the CEK until the TTL expires.
5.5 Plasma Proxy Servers
There are two separate use cases for proxy servers in Plasma. The
forward proxy use case where a DR client needs to connect to a PDEP
outside of its organization and the reverse proxy use case where a DR
client outside an organization needs to connect to a PDEP.
A recipient has no control over senders creating Plasma email (or any
other type of Plasma protected content) and sending it to them.
Malicious senders can craft harmful payloads and protect it in a
Plasma envelope. Therefore, Plasma recipients need a policy to
determine the set of Plasma PDEP services they are willing to interact
with. This can be a local policy, i.e., a policy for the allowed set
of PDEPs a DR client can interact with. This policy would need to be
distributed to every DR client. An alternate approach is to have a
forward proxy manage the policy on behalf of the DR client. A forward
proxy would eliminate the need to distribute policy by mediating the
connection requests from the DR clients to the PDEP services. The
forward proxy could be a server belonging to the DR client
organization or a cloud service.
In the no-proxy use case the DR client would connect via TLS directly
to the URL contained in the policy metadata. The DR would thus need
local policy to determine whether to connect to the PDEP URL. If a
forward proxy is preset, the DR client would attempt to connect via
TLS to the forward proxy. The forward proxy would then connect to the
PDEP if its policy allowed.
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Internet | DMZ | Intranet
| |
| |
| | ---------------
| | | |
TLS | | TLS | DR |
----------|<------------------------|------| Client |
| | | |
(a) | | ---------------
no proxy | |
| |
| |
| --------------- | ---------------
| | | | | |
TLS | | Plasma | | TLS | DR |
----------|<-----| Forward |<---|------| Client |
| | Proxy | | | |
(b) | | | | ---------------
Forward | --------------- |
Proxy | |
Figure 4 Plasma Forward Proxy
Since the Plasma service has sensitive cryptographic keys used to
protect the data CEKs, it would be unwise to host those servers
directly connected to the Internet. However, PDEPs will need to be
Internet addressable for requests from DR clients outside the
organization. The simplest possible configuration would be to have a
passive reverse proxy in front of the Plasma server. Since Plasma is
using TLS, a passive proxy cannot inspect the data inside the TLS
session. The passive proxy has therefore a limited function and would
be only able to filter based on session characteristics, e.g., source
IP addresses. The Plasma protocol is a series of request-response
messages, so an active reverse proxy can be implemented like other
store-and-forward message-based services, e.g., SMTP. The Internet-
facing proxy server would terminate the TLS connections from the
external DRs. The active proxy can then scan submitted requests to
ensure they are not malformed and are free from malicious content
before relaying messages to a full PDEP server further inside the
network for processing of the request.
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Internet | DMZ | Intranet
| |
| |
| --------------- | ---------------
| | | | | |
TLS | | Passive | | TLS | Full |
----------|------|-------------|----|----->| PDEP |
| | Reverse | | | Server |
| | Proxy | | | |
(a) | | | | | |
| --------------- | | TLS Keys, |
| | | Message |
| | | Encryption |
| | | Keys |
| | | |
| | ---------------
| |
| --------------- | ---------------
| | | | | |
TLS | | Active | | TLS | Full |
----------|----->| Reverse |----|----->| PDEP |
| | Proxy | | | Server |
(b) | | | | | |
| | TLS keys | | | TLS Keys, |
| | | | | Message |
| --------------- | | Encryption |
| | | Keys |
| | | |
| | ---------------
| |
| |
Figure 5 Plasma Reverse Proxy
5.6 Policy Types
Policies range from very simple to very complex. Policies have
dependencies not only on the technical implementation of the software
but on the range of attributes a PIP would issue to subjects. This is
likely constrained by the physical procedures a PIP could support to
capture and verify the information about the subject. To manage this
range of requirements, this model uses two type types of policy.
5.6.1 Basic Policies
Basic policies are intended to be universally usable by employing a
small, fixed set of attributes that are available from all PIPs. For
example, basic policies are intended to be equivalent to sending
encrypted email with S/MIME today, i.e., authenticated recipients of
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the email get access to the message. Basic policies target scenarios
involving consumers and small businesses who are using public PIPs
which issue a limited set of attributes. It is expected that all
Plasma clients and commercial IdPs would be capable of supporting
basic policies due to the finite set of attributes required which will
simplify development, testing, and deployment. Later standards may
expand the set of attributes supported by basic policies and hence
define richer basic policies.
5.6.2 Advanced Policies
Advanced policies are intended to be used where one or more policies
are required on the content that require an expanded set of attributes
from a PIP. They are intended to target more complex policy
requirements such as content with regulated information or content
subject to organizational and contractual policies. The input set of
attributes are defined by the policies. These attributes are, in
theory, unbounded and can be either primordial such as date of birth,
or derived attributes such as age, or both. In practice, advanced
policies are constrained by the set of attributes available under the
IdP Trust Framework for the subjects. A data object may require
multiple policies and any instance of multiple policies requires a
logical relationships between the policies, e.g., they can be AND-ed
or OR-ed together. It is not expected that all Plasma clients will
support the rich set of attributes necessary for advanced policies.
6 Message Protection Requirements
6.1 General Requirements
Confidentiality policy-protected messages MUST be protected from
unauthorized disclosure, protected from unauthorized alteration, and
provide data origin authentication.
Integrity policy protected messages MUST be integrity protected from
unauthorized alteration and provide data origin authentication.
The specifics of every possible authentication mechanism or every
detail about how the subject's identity was proofed by the IdP cannot
be known to the DR and PDEP, therefore the specifics of how the sender
or recipient achieves the required level of identity assurance SHOULD
be abstracted from the PDEP and DR by use of a simple numeric scale
linked to an identity assurance framework that defines the specifics
of how to derive the LoA. (See sections 4.1, 4.2, 4.3 and 4.4.)
Access policies are complex and subject to change over time. For this
reason, policies MUST be identified by reference rather than inclusion
of the actual policy with the message so the policy changes can be
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implemented without updating the message. (See section 4.4.)
Access to the plaintext of the message MUST only be provided after the
recipient has either provided suitable valid attributes to the PDEP or
the PDEP finds attributes about the recipient directly from a PIP,
thus satisfying the policy as defined by the PAP. (See sections 4.1,
4.2, 4.3, 4.4 and 4.5.)
The sender MUST be able to obtain a list of policies to messages they
create and scoped to their current role(s), i.e., what tasks they are
currently assigned to deliver. (See sections 4.1, 4.2, 4.3 and 4.4.)
The specifics of the access control policy MUST be abstracted from
both the sender's and the recipient's, i.e., they MUST NOT make the
access control decision or need specifics of the access policy
requirements. (See sections 4.1, 4.2, 4.3 and 4.4.)
A recipient MUST receive authenticated attributes of the identity and
level of identity assurance of the sender. (See sections 4.1, 4.2, 4.3
and 4.4.)
The key exchange between sender, recipient, and the PDEP MUST support
multiple levels of assurance of the exchange. For example, for low-
assurance situations this could be via a plan text CEK over a secure
transport such as TLS. For high assurance situations, the recipient
is required to provide a suitable key exchange key such as an X.509
certificate to encrypt the CEK. (See sections 4.3 and 4.4.)
The level of key exchange assurance required MUST be selected by the
sender's policy and enforced by the PDEP. (See sections 4.1, 4.2, 4.3
and 4.4.1.)
If the recipient is unable to initially comply with the sender's
policy, then is subsequently able to get the required credentials or
attributes, it MUST be possible for the recipient to retry access to
the content without intervention from the sender.
A time-to-live (TTL) MUST be provided to recipients when access is
granted to a message by the PDEP to define when the recipient MUST
discard the message CEK and submit a new access request to the PDEP.
The TTL value MUST be based on the message policy and optional
attributes about the recipient and its environment. (See section
4.4.1.)
The PDEP MUST be stateless for processing policy requests from senders
and recipients with respect to any instance of a message. It MUST be
possible to have multiple instances of a PDEP service and load balance
requests across all instances of the service transparently to the
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client and not require synchronization of state about requests between
instances of the service. (See section 4.9.)
A PDEP MUST be capable of generating audit events associated with
access to protected content using policy defined by the PAP. (See
section 4.6.)
It MUST be possible for domains to publish keys and attributes about
the boundary inspection agents. This allows senders to pre-authorize
the inspection agents of recipients for access to messages. (See
sections 4.7 and 4.8)
It MUST be possible for MTAs to request access to protected messages
for which they have not been authorized by the sender. (See section
4.7 and 4.8.)
It SHOULD be possible for an MTA to pre-authorize another to access a
protected message. (See section 4.7 and 4.8.)
6.2 Basic Policy Requirements
The use of a Basic policy MUST be backwards compatible with existing
S/MIME, i.e., it MUST be possible to both to exist on the same
message.
A sender's agent MAY discover some recipients' encryption certificates
and create recipient info structures using the existing S/MIME
standard (unless specifically forbidden by the selected policy).
A sender's agent MAY elect to use a Basic Policy mechanism for
recipients for whom encryption certificates cannot be discovered.
Four Basic policies are to be defined by this work. These Basic
policies MUST map to the LoA of NIST 800-63-1. This does not preclude
the definition of other Basic policies to be defined by other groups,
trust frameworks, or even within the context of the IETF.
When using a Basic policy defined by this work, the sender MUST define
which Basic policy is required and the list of [RFC5322] recipients.
The list of recipients MUST be able to be appended to by a
distribution list expansion server.
A sender using a Basic policy MUST be able to send protected messages
without discovering a recipient's encryption key.
A sender using a Basic policy MUST NOT require a bilateral agreement
between sender and recipients as a prerequisite to sending the
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message.
6.3 Advanced Policy Requirements
A Basic policy MAY be combined with Advanced policies on a message
The use of an advanced policy and existing S/MIME confidentiality on
the same message MUST NOT be permitted.
It MUST be possible to apply one or more Advanced policies to a
message.
Where two or more policies are applied to a message, the logical
relationship between the policies MUST also be expressed, e.g., are
the policies a logical AND or a logical OR. (See section 4.4.)
An Advanced policy MAY require attributes about:
o The recipient
o The recipient's device
o The environment of the device that is attempting to access the
message.
o The message being accessed
Advanced policy MUST support an extensible list of obligations on the
sender or recipient or PDEP such as use of the policy requires some
specific action on the part of the sender, e.g., signing content with
a two-factor smart card and/or that the signature complies with the
legal requirements for the transaction, or the signature needs to be
able to be verified for an extended period.(See sections 3.3 and 3.4.)
Advanced policies MUST support the ability to verify the content for
an extended period as required by policy. For example policy may
require signatures to be verifiable for a period of 10 years.
Advanced policies MUST support the ability to resign the data to
support the verification over the extended period.
7 IANA Considerations
This document describes the requirements for message access control.
As such, no action by IANA is necessary for this document
8 Security Considerations
Authentication by itself is not a good trust indicator. Authentication
raises the level of assurance that the identity is correct but does
not address whether the identity is trustworthy or noteworthy to the
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recipient. Authentication should be coupled with some form of
reputation, e.g., the domain is on a white list or is not on a black
list. Malicious actors may attempt to "legitimize" a message if an
indication of authentication is not coupled with some form of
reputation.
Malicious actors could attempt to use encrypted email as a way to
bypass existing message pipeline controls or to mine information from
a domain. Domains should have sufficient granularity of policy to
handle situations where their email pipeline agents are not able to
inspect the contents.
It must be possible for a third party to, upon correctly presenting a
legitimate legal justification, to recover the content of a message.
This includes the sender's and recipient's companies for business
continuity purposes, as well as law enforcement. If the entity
requesting the information and the entity controlling the access are
in different jurisdictions, then the process would be subject to some
form of rendition.
The use of a security label type that requires the recipient of a
message to query a PDEP in order to obtain the contents of a message
opens an additional method for adversaries to confirm that an email
address does or does not exist.
Additionally, it allows for a new channel for materials to be
delivered to the recipient's mail processor that is not checked for
malware or viruses by the standard mail scanning methods in place.
Email is frequently used as part of a password reset ceremony by an
identity provider. This is problematic when combined with access to
sensitive email. This could be part of an escalation attack, e.g.,
compromise low value email account password, initiate password reset
via email for higher value account. This would then give access to any
email protected using the higher value identity.
Providing differential access to different parts of a message based on
different policies should only be done via use of different encryption
keys. All data protected by the same key is under the same access
control policy.
It would be desirable to be able to indicate the times and other data
like request location when a user has asked for access (successful or
otherwise) to some content as a means to show malicious activity to
the user.
Part of the policy is obligations on how to protect the data, e.g.,
algorithms and parameters required. This can change over time,
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therefore a client may become obligated to re-encrypt or re-digest the
data if it encounters data which does not meet the current mandate.
The act of requesting access to messages is a potential privacy issue
as it allows the sender to gather data about the recipient. For
business-to-business transactions, disclosure of employee information
is handled by the organization. For consumers, there is a need to be
able to consent to the privacy obligations associated with disclosure
of information. This would include information the consumer releases
to the PDEP as well as information the PDEP is able to gather such as
time and location of access requests.
The fact the PDEP is able to grant access to the data could be used by
law enforcement to access information. One of the parameters the
sender needs to be aware of is the jurisdiction the PDEP is under so
they can make an informed choice.
Appendix A. References
A.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3198] Westerinen et. al., "Terminology for Policy-Based
Management", November 2001.
[RFC5035] Schaad, J., "Enhanced Security Services (ESS) Update",
August 2007.
[RFC5280] Cooper, D, et al, "Internet X.509 Public Key
Infrastructure Certificate and Certificate Revocation
List (CRL) Profile", RFC5280, May 2008
[RFC5322] Resnick, P., "Internet Message Format", RFC5322, October
2008.
[RFC5652] Housley, R., "Cryptographic Message Syntax (CMS)", RFC
5652, September 2009.
[RFC5750] Ramsdell, B. and S. Turner, "Secure/Multipurpose Internet
Mail Extensions (S/MIME) Version 3.2 Certificate
Handling", RFC 5750, January 2010.
[RFC5751] Ramsdell B., Turner S., "Secure/Multipurpose Internet
Mail Extensions (S/MIME) Version 3.2 Message
Specification", January 2010
[SAML-core] OASIS, Assertions and Protocols for the Security
Assertion Markup Language (SAML) Version 2.0, March 2005
A.2. Informative References
[RFC3114] Nicolls, W., "Implementing Company Classification Policy
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with the S/MIME Security Label", RFC 3114, May 2002.
[RFC5408] Appenzeller, G., "Identity-Based Encryption Architecture
and Supporting Data Structures", RFC5408, January 2009.
[XACML-core] OASIS, eXtensible Access Control Markup Language (XACML)
Version 3.0 Core Specification
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Appendix B Authors' Addresses
Trevor Freeman
Email: trevor.freeman99@icloud.com
Jim Schaad
Soaring Hawk Consulting
Email: ietf@augustcellars.com
Patrick Patterson
Carillon Information Security Inc.
Email: ppatterson@carillon.ca
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